I had exactly the same problem, and I also had an indirect hot water heater. My indirect heater consisted of a coil of copper pipe inside the boiler, which circulated hot boiler water through a storage tank, where the domestic water was heated. In my case, there was a small leak in the coil inside the boiler. Since the pressure in the indirect coil is house pressure (50 psi in my case), and is not controlled by the boiler makeup water pressure regulator, the effect of a leak in the coil is to pressurize the water in the boiler to house pressure.
I could have replaced the coil, which would have cost about $700, and came with no guarantees, because my boiler is old (1959), and the bolts holding the coils to the boiler are rusty and suspect. I chose to purchase an electric hot water heater and dismantle the indirect system. I capped off the coil lines where they enter the boiler. Problem solved.